1. Field of the Invention
The present invention relates to an electric power supply system which supplies a power generated by an AC generator to a load, and more particularly to a system which is suitably mounted on a vehicle to supply an electric power to a battery and other loads.
2. Description of the Related Art
In a vehicle, particularly, in a motorcycle or the like, an electric power supply system which uses a synchronous generator and a short-circuit type regulator is employed from the viewpoints of miniaturization, cost reduction, and the like. As shown in FIG. 8A, for example, such a system includes: a synchronous generator 101; diodes D101 and D102 and a capacitor C101 which constitute a rectifying circuit; FETs (Field Effect Transistors) Q101 and Q102 and diodes D103 and D104 which constitute a switching circuit for performing a voltage control; a control section 102 which performs a switching control on the FETs; a battery 103; and an electrical load 104. The control section 102 monitors an output voltage VRCT of the rectifying circuit, and, when the output voltage exceeds a predetermined upper limit voltage VHL, outputs a switching signal SW which causes the FETs Q101 and Q102 to be turned on. In this configuration, when the FETs Q101 and Q102 are turned on, the circuit of FIG. 8A has a state in which the output terminals of the generator 101 are equivalently short-circuited as shown in FIG. 8B, thereby preventing the output voltage VRCT from rising above the upper limit voltage VHL.
FIGS. 9 and 10 are time charts illustrating the operation. In the figures, for the sake of convenience in description, changes of the voltage and the current in a state where the capacitor C101 is eliminated. As the rotational speed NACG (rpm) of the generator 101 is raised, the voltage VRCT is raised. When the output voltage reaches a voltage VBAT, charging of the battery 103 is started. When the output voltage VRCT is further raised to reach the upper limit voltage VHL, the FETs Q101 and Q102 are turned on and the output voltage VRCT becomes xe2x80x9c0xe2x80x9d. When the number of revolutions of the generator reaches a usually used number so as to attain a stationary state, the voltage VRCT and the switching signal SW become as shown in FIG. 10. Actually, the output voltage VRCT of the rectifying circuit is maintained to a substantially constant level by the function of the capacitor C101 and the current output from the battery 103.
From a broad perspective, the above-mentioned operation seems to be equivalent to a situation in which the voltage VRCT is maintained to a constant level by controlling an average load resistance RLV which is connected to the output of the rectifying circuit 105 in parallel with the battery 103, and the like as shown in FIG. 8C.
In a conventional electric power supply system which uses a short-circuit type regulator such as shown in FIG. 8 and which is employed in a motorcycle or the like, the output characteristic at the idling rotation of an engine which drives the generator 101 may be set to the charging voltage VBAT which is necessary for charging the battery 103. In this case, when the engine rotates at a high number of revolutions, a power which is larger than that required for charging the battery 103 is generated, and the output voltage Vt of the generator 101 exceeds the upper limit voltage VHL. When the voltage is raised, therefore, the output terminals are short-circuited, whereby the average load resistance RLV is lowered so that the output voltage of the rectifying circuit is maintained to a level which is slightly higher than the charging voltage VBAT. In other words, when the output voltage Vt is raised, the output terminals are short-circuited to equivalently lower the load resistance, and an unwanted power is dissipated, thereby maintaining the voltage to a constant level.
FIG. 11 is a characteristic diagram which shows variations of the output power P and the output current I and in which the abscissa indicates the output voltage Vt of the generator 101. The broken lines L1 and L2 correspond to characteristics during idling of the engine (at the rotational speed NACG=f1), and the solid line L3 and L4 correspond to characteristics at a high number of revolutions (at NACG=f2 greater than f1). The conventional voltage controlling technique described above corresponds to the case where, at a low number of revolutions, RLV=R1 is set and the operating point (I=I1, Vt=VCNST) is at an intersection of the straight line of an inclination of 1/R1 and the broken line L2, and, at a high number of revolutions, RLV=R2 is set and the operating point is moved to an intersection (I=I2, Vt=VCNST) of the straight line of an inclination of 1/R2 ( greater than 1/R1) and the solid line L4. In the conventional controlling technique, therefore, the voltage can be maintained to a constant level, but a heat loss occurs as a result of the short circuiting and hence the generator wastefully generates a power, thereby causing a problem in that energy is largely lost.
The invention has been conducted in view of the problem. It is an object of the invention to provide an electric power supply system in which the operating point of an AC generator can be appropriately controlled and the energy loss can be suppressed to a minimum level.
In order to attain the object, according to a first aspect of the invention, in electric power supply system for supplying a power generated by an AC generator to a load, the system comprises controlling means, disposed between the load and the AC generator, for performing a control so that the AC generator operates in a current range which is lower in level than an output current corresponding to a maximum power operating point of the AC generator.
In this configuration, the AC generator is controlled so as to operate in a current range which is lower than an output current corresponding to the maximum power operating point of the AC generator. Therefore, the energy loss due to the internal resistance of the AC generator can be suppressed to a minimum level, with the result that an electric power supply system of a high efficiency can be realized.
According to a second aspect of the invention, in the power supply system of the first aspect of the invention, the AC generator has a drooping characteristic in which, as the load is increased, an output voltage is lowered and an output power is increased, the output power is maximum at the maximum power operating point, and, when the output voltage is further lowered, the output power is reduced, and the controlling means performs, a control so that a load resistance of the AC generator starts from an initial state in which the load resistance is substantially infinite, and is reduced with a passage of time.
In this configuration, the load resistance of the AC generator having a drooping characteristic is controlled in such a manner that the load resistance starts from an initial state in which the value is substantially infinite, and is then reduced with the passage of time. Therefore, an operation of the AC generator at a desired operating point can be surely realized by a relatively simple control.
According to a third aspect of the invention, in the power supply system of the first or second aspect of the invention, the controlling means has rectifying means for rectifying an output of the AC generator, and DC voltage converting means for lowering an output voltage of the rectifying means and then supplying the output voltage to the load, and performs a feedback control so that an output voltage of the DC voltage converting means coincides with a target voltage.
In this configuration, the output of the AC generator is rectified, and feedback controlled so that the DC voltage applied to the load coincides with a target voltage. Therefore, the energy loss of the AC generator can be suppressed to a minimum level, and, even when the output of the AC generator is varied, a stabled DC voltage can be always supplied.